Usually, in order to carry out an optical recording and reproduction on an optical disk such as a CD or a DVD, various kinds of optical pickup devices have been developed. The optical pickup device projects a light from a light source to the optical disk by using an objective lens and guides a reflected light from the optical disk to a light receiving element to read various kinds of information recorded on the optical disk. The optical pickup device has been known in which a light flux separating unit (for instance, a beam splitter or a half plate mirror) is provided between the light source and the light receiving element to separate optical paths of a projection optical system and a light receiving optical system (for instance, see Patent Document 1).
That is, as shown in FIG. 14, this optical pickup device includes a light flux separating unit such as a half plate mirror (abbreviate it as a “half plate”, hereinafter) 102 in the intermediate part of an optical path from a light source 101 to an optical disk D. In FIG. 14, reference numeral 103 designates a collimate lens, 104 designates a rise mirror, 105 designates an objective lens, 106 designates a parallel flat plate and 107 designates a light receiving element.
Patent Document 1: JP-A-2002-123969